Conventionally, an electrode of an electrode drawing portion in a wound plastic film capacitor having a self-security mechanism is formed to be thick. This increases strength of contact with a metalized contact which is an electrode introducing portion. Such a film capacitor is disclosed in Japanese Patent Unexamined Publication No. S62-183506.
There has been known a self-security mechanism in which microscopic blocks are formed in a deposited electrode using slits in which metal is not deposited, and fuses provided between the slits are connected between the microscopic blocks, with divided electrodes interconnected in parallel. There has been also recently known a self-security mechanism that employs a divided electrode pattern having slits provided in the form of a lattice. In addition, Japanese Patent Unexamined Publication No. 2004-134561 discloses a self-security mechanism pattern having the characteristic that fuses emit little heat and the amount of reduction of their capacitance is low when current flows.
FIGS. 6A to 7B are schematic views showing conventional metalized film capacitors. FIG. 6A is a perspective plan view of a conventional metalized film capacitor, FIG. 6B is a sectional view taken along line C-C′ of FIG. 6A, FIG. 7A is a perspective plan view of another conventional metalized film capacitor, and FIG. 7B is a sectional view taken along line D-D′ of FIG. 7A.
In the structure shown in FIGS. 6A and 6B, high resistance portions 12A and 12B and low resistance portions 13A and 13B, both of which are formed of metal deposition films, are respectively provided on dielectric films (hereinafter abbreviated as films) 11A and 11B such as polypropylene films. First slits 16B, second slits 16A and third slits 16C are provided in high resistance portions 12A and 12B. Portions between the slits function as fuses 14. Microscopic blocks 17 are surrounded by the slits.
A metalized contact (not shown) as an electrode drawing portion is welded to low resistance portions 13A and 13B at the sides of films 11A and 11B. Accordingly, low resistance portions 13A and 13B are formed to be thicker than high resistance portions 12A and 12B, and has low film resistance. Band-shaped portions 15A and 15B, on which metal is not deposited, are provided as insulation margins on films 11A and 11B such that short-circuit between the metal deposition films on films 11A and 11B does not occur when the metalized contact is welded. First slits 16B, second slits 16A and third slits 16C are provided at the center in the width direction of films 11A and 11B or at a side closer to band-shaped portions 15A and 15B than the center.
In the structure shown in FIGS. 7A and 7B, an electrode pattern in high resistance portions 12C and 12D is different from that shown in FIGS. 6A and 6B. That is, first slits 16B, second slits 16A and third slits 16C are provided in the entire width direction of high resistance portion 12C, and no slit is provided in high resistance portion 12D.
Fuses 14 function as a self-security mechanism in the metalized film capacitor shown in FIGS. 6A and 6B. However, when films 11A and 11B, in which the slits are provided in this manner, are laminated to form the capacitor, fuses 14 provided at the center in the width direction of laminated films 11A and 11B overlap with each other. That is, first slits 16B provided at upper film 11A and lower film 11B overlap with each other. Accordingly, fuses 14 provided in first slits 16B overlap with each other, too. A good deal of heat is locally generated in this overlap portion when large current flows.
On the other hand, in the metalized film capacitor shown in FIGS. 7A and 7B, fuses 14 are provided in the entire of high resistance portion 12C, and fuses 14 are also formed at a side closer to an electrode drawing portion than the center in the width direction of film 11A. The heat generation at fuses 14 on the side of the electrode drawing portion is large.
The conventional film capacitors having the structure as described above generate a lot of heat, which results in reduction of the lifetime of the capacitor and non-uniformity in operation of fuses 14 as the self-security mechanism.